Photoengraving resists and compositions therefor



United States Patent 3,366,481 PHOTQENGRAVING RESESTS AND COMRGQTEONS THEREFOR Murray J. Laione, Cluster, N.J., assignor to Harniiclt Research & Development Corporation, Cluster, N..l., a

corporation of New Jersey No Drawing. Filed Sept. 20, 1963, Ser. No. 310,483 18 Claims. ('Cl. 96-35) This invention relates to photosensitive resists and compositions for their production. More particularly, it is concerned with photoengraving plates of the class in which a light sensitizing agent is dispersed in a water soluble, photosensitive polymeric layer on a zinc or magnesium base, and with compositions useful for their production.

Photoengraving plates for use in letter press printing are now in general use. For many years these plates were prepared by coating a metal base, for example, zinc, magnesium or other etchable metal, with a relatively thin layer of a natural polymer such as albumen or gelatin in which a light sensitizing agent was incorporated. The principal light sensitizing agent for use with these plates is hexavalent chromium in the form of a dichromate salt. An image is produced on such a plate by exposure to a source of ultraviolet light which actuates the light sensitizing agent, which in turn causes the exposed portion of the photosensitive layer to harden to an insoluble polymer probably by free radical initiated polymerization. The unexposed portion of the layer is then dissolved by washing with water to leave the hard polymer image.

More recently, the albumen or gelatin layer has been replaced with synthetic polymeric materials which are polymerized further by the action of the light sensitizing agent to produce a hard water insoluble image. Water soluble, commercial polyvinyl alcohol is an example of such a synthetic polymer. Polymerization to a hardened image is catalyzed by dichromate light sensitizing agents, preferably water soluble dichromate salts such as ammonium dichromate or alkali or alkaline earth metal dichromates.

In the preparation of a photoengraving plate suitable for use in letter press operations a zinc or magnesium plate is thoroughly cleaned and a photosensitive layer comprising a water soluble polymeric material with the light sensitizing agent dispersed therein is deposited on the surface. Several methods are available for depositing the thin photosensitive layer. In one convenient method, a solution containing the appropriate water soluble polymer and light sensitizing agent is deposited on the surface of the plate and spread into a thin layer by whirling the plate. The application of heat during the swirling operation evaporates the solvent so as to leave a dry polymeric layer containing the dispersed sensitizing agent throughout the film. For special purposes, other materials such as dyes may be incorporated. The photoengraving plate with its photosensitive layer is then exposed to light through a photographic negative of the image which is to be ultimately produced. This is preferably accomplished by placing the negative over the plate in a vacuum frame. The vacuum serves to hold the negative in close contact with the plate. While in contact, the plate is exposed to a source of ultraviolet light, usually a carbon arc, which light impinges on the plate through the transparent parts of the negative. Generally, the plate is exposed to a double carbon arc lamp rated at 35 amps at a distance of three to four feet from the negative for a period of three to eight minutes. The light sensitizing agent in the exposed portions of the plate is activated to initiate further polymerization of the polymer so as to produce a hardened, water insoluble polymeric image corresponding to the image on the negative.

The plate is then developed by dissolving unexposed portions of the film, for example, by spraying with water. In the next step, the image is fixed by depositing chromic acid in the film which is hardened by subsequent heating. This may be accomplished, for example, by dipping the plate into a 6 to 8% chromic acid solution for approximately twenty seconds. The excess acid is removed with water. The plate with the fixed image is then dried, preferably in hot air, and baked in an oven at approximately 350 F. to 450 F. to further develop resistance of the hardened polymeric image to the etching chemicals. The plate is then further washed or descummed and finally etched. Zinc plates may be etched, for example, with 10 to 15% nitric acid solution which may contain solvents and wetting agents. Magnesium plates may be etched with similar solutions containing 9 to 11% nitric acid.

In the past, it has not been possible to do exceptionally fine photoengraving work such as four color letter press printing utilizing 133 line screens in the production of the polymeric image except with copper plates. Even in these instances it has been necessary to utilize extremely tedious and expensive techniques, and in many cases it has only been possible to attain high quality reproduction by hand techniques. It has, accordingly, long been a problem in the art to provide a method by which zinc and magnesium plates, which are much cheaper than copper plates and much easier to work with, can be utilized with 133 line screens without the necessity of exceptionally careful workmanship and control techniques.

Moreover, undercutting of the polymeric image has been a particularly difiicult problem with all base metals. The image is comprised of an extremely large number of dots and/ or lines which may be viewed as pillars or ridges of the base metal, each of which is capped with a polymer which is resistant to the etching solution. During the etching step the pillars or ridges, or sections of them, are dissolved away from the underside of the polymeric cap with the result that the cap itself is easily broken away by the mechanical action of printing so that it is often impossible to produce commercially acceptable photoengravings. Despite newer techniques with respect to the etching solutions and the use of modern synthetic polymers, this problem has continued to plague the industry.

In accordance with this invention, I have found that with photoengraving plates produced using aqueous compositions containing critical amounts of polymeric mixtures such as polyethylene maleic anhydride, partiallyneutralized polymethacrylates or polyacrylic acid in a specific molecular weight range in admixture with a highly purified polyvinyl alcohol, the tendency of the caps to break away under stress is reduced to such an extent that it is possible to utilize zinc or magnesium plates in opera tions which have heretofore required copper plates. More over, the plates can be used in accordance with standard techniques without the necessity of hand operations.

The water soluble polyacrylic acids are preferred and as used in preparing the compositions for the photosensitive layers of my invention are in the molecular weight range from about 25,000 to about 150,000. I prefer to utilize the acids in the range of 25,000 to 50,000, since with samples of a molecular weight greater than 50,000 a longer washing period is required to remove the unexposed portions of the photosensitive layer. For example, if the plate is washed with ordinary tap water at domestic pressure, the washing is complete in approximately one minute if the molecular weight of the polyacrylic acid is from 25,000 to 50,000. However, with acids of molecular weight above this preferred range the washing time is somewhat increased. The hardened images produced are substantially the same no matter what the molecular weight of the acid, provided it is within the aforementioned broad operating range.

The molecular weight range of the polyvinyl alchols used in this invention is the same as the range which has heretofore been employed when utilizing polyvinyl alcohols in the preparation of photoengraving plates. The purity of the polyvinyl alcohol used in this invention, however, is critical. It is essential that the ash content attributable to polyvinyl alcohol in the final photosensitive layer be no higher than about 0.6% by weight. Above this critical amount a soft image results.

A convenient test to determine the suitability of a particular polyvinyl alcohol sample for use in this invention is to prepare a 10% aqueous solution and measure the pH. If the pH is above about 5.9 the polyvinyl alcohol has too high an ash content and is not suitable.

The preferred molecular weight of polyvinyl alcohols used in this invention is such that a 4% equeous solution has a viscosity of from about 4 to 6 centistokes measured at 20 C. Other polyvinyl alcohols somewhat above or below this range can also be employed provided, however that the ash content is not higher than about 0.6%.

The compositions of this invention are prepared by dissolving the polymeric mixture in water to form a solution which is approximately 23-20% by weight in polymer. The amount of polyacrylic acid employed for use with zinc plates is from about to about by weight based on the total amount of polymer. With magnesium plates the amount is from about 5% to about 6.7% and the pH of the composition is adjusted to approximately 5.5 to 6.0 by the addition of a water soluble base. Ammonium hydroxide is the preferred base, but other water soluble nitrogenous bases such as pyridine, morpholine and piperidine can also be employed. Metallic bases such as sodium hydroxide can be used but it is preferred not to do so since the fixed base accumulates in the photosensitive layer and may be attacked by the etching solution.

The standard light sensitizing agents employed with ordinary work may also be utilized in my invention, and in substantially the same quantities. For example, dichromate light sensitizing agents can be used. Ammonium dichromate or alkali metal dichromates are suitable, but the preferred sensitizing agent is chromic acid prepared by adding magnesium hydroxide to an aqueous solution of chromium trioxide. With compositions intended for use with zinc plates, I prefer to have a dichromate content of approximately 0.25 to 0.75% by weight. For magnesium plates, the compositions preferably contain 0.5 to 0.7% by weight. The amounts of dichromate in the final photosensitive layers are then 2.5 to 7.5% by weight for zinc and 5% to 7% for magnesium. The pH of the preferred sensitized compositions for use with zinc is from about 3 to 4 and for magnesium about 4.6 to 5 .0.

A mold inhibitor may also be employed to help preserve the film from deterioration due to mold action. Dyes may be added to the composition in accordance with the usual techniques.

A typical composition of this invention for use on a zinc base may be prepared as follows:

To 665 lbs. of water (80 gallons) in a large kettle there is added 80 lbs. of a polyvinyl alcohol which in 4% aqueous solution has a viscosity of 4 to 6 centistokes at C. The temperature is brought to 200 F. with good stirring, held for two hours and the solution cooled. A total of eight pounds of polyacrylic acid with a molecular weight of about 50,000 is then added. Alternatively, the polyacrylic acid can be added before heating.

If desired, the light sensitizing agent may be added at this stage. It is preferred, however, to add the sensitizing agent just prior to use. This increases the shelf life of the composition. It has been found that compositions con- A. taining light sensitizing agents deteriorate to some extent on long standing.

The viscosity of the solution prepared as described above is from about 30 to about 40' centistokes at room temperature. I prefer to add sufiicient water to adjust the viscosity to about 18 to 25 centistokes before the composition is packaged for use. This is not essential, however, since the viscosity may be adjusted at any time to provide a composition having physical properties best suited to the manner of forming the photosensitive layer on the zinc plate.

As aforesaid, the sensitizer can be added just prior to use. A typical sensitizing composition may be prepared by dissolving 21 lbs. of chromium trioxide in 25 gallons of water and then adding 6.7 lbs. of magnesium hydroxide. If six fluid ounces of this solution is added to 122 fiuid ounces of the polymeric solution prepared as described above and adjusted to have a viscosity of between 18 and 25 centistokes, the resulting solution will have a viscosity between about 13 and 20 centistokes and be suitable for forming a photosensitive layer on a zinc plate in accordance with the spin technique described above.

Typical compositions of this invention for use with magnesium plates may be prepared as follows:

To 665 lbs. of water are added lbs. of polyvinyl alcohol which in 4% aqueous solution has a viscosity of 4-6 centistokes at 20 C. The temperature is brought to 200 F. with good stirring, held for two hours, cooled and 5.3 lbs. of polyacrylic acid with a molecular weight of about 50,000 are added. The polyacrylic acid can be added before heating. The solution is then adjusted to a pH of 6.0 by the addition of 4.3 lbs. of 265 Baum ammonium hydroxide or 5.5 by the addition of 3.6 lbs. of the same reagent.

The viscosity of the solution thus prepared is from about 25 to about 35 centistokes at room temperature and may be adjusted to about 18 to 25 centistokes if desired.

The sensitizer, as before, can be added during the preparation or just prior to use. With a solution adjusted to a pH of 5.5 a useful sensitizer may contain 26 lbs. of chromium trioxide and 5.65 lbs. of magnesium hydroxide in 27 gallons of water. Six fluid ounces of this solution added to 122 fluid ounces of the above polymeric solution having a viscosity of between 18 and 25 centistokes will produce a sensitized solution with a viscosity of approximately 13 to 20 centistokes and a pH of 4.6.

With a solution adjusted to a pH of 6.0 a preferred sensitizing solution contains 26 lbs. of chromium trioxide and 4 lbs. of magnesium hydroxide in 27 gallons of water. Six fluid ounces of this solution added to 122 fluid ounces of the above polymeric solution having a viscosity between 18 and 25 centistokes will also produce a sensitized solution with a viscosity of approximately 13 to 20 centistokes. The pH is approximately 5.0.

I prefer to operate with final compositions having a viscosity between 16 and 18 centistokes at room temperature and to form the photosensitive layer in the thickness range of from about 0.0002 and about 0.0004 inch. I prefer to deposit the layer by whirling for about 3-5 minutes at a temperature of about F. to about F., although the use of heat is not essential.

Photoengraving plates prepared with the compositions of this invention are utilized in exactly the same manner as ordinary plates, but with remarkably improved results. Images prepared through 133 line screens with ordinary zinc or magnesium plates prepared from a composition of this invention by the usual procedures are as acceptable for commercial use as copper plates prepared by hand. These results are achieved with compositions prepared in accordance with the examples specifically set forth above and with other compositions within the purview of the foregoing disclosure.

While the invention has been described in detail with respect to preferred embodiments, it will be understood by those skilled in the art that various modifications may be made without departing from the spirit and scope of the invention, and it is intended to cover all such changes and modifications in the appended claims.

What is claimed is:

1. An aqueous composition comprising a polymeric solution containing from about 8% to about 20% by weight of a polymeric mixture of water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight together with from about 5% to about 10% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 150,000.

2. An aqueous composition comprising a polymeric solution containing from about 8% to about by weight of a polymeric mixture of water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight together with from about 5% to about 10% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 50,000.

3. An aqueous composition comprising a polymeric solution containing from about 8% to about 20% by weight of a polymeric mixture of water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight together with from about 5% to about 10% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 150,000 and a light sensitizing agent.

4. An aqueous composition comprising a polymeric solution containing from about 8% to about 20% by weight of a polymeric mixture of water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight together with from about 5% to about 10% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 50,000 and a light sensitizing agent.

5. An aqueous composition comprising a polymeric solution containing from about 8% to about 20% by weight of a polymeric mixture of Water soluble polyvinyl alcohol having an ash content of less than 0.6% by Weight together with from about 5% to about 10% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 150,000 and a dichromate light sensitizing agent.

6. An aqueous composition comprising a polymeric solution containing from about 8% to about 20% by weight of a polymeric mixture of water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight together with from about 5% to about 10% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 50,000 and a dichromate light sensitizing agent.

7. An aqueous composition having a pH of from about 5.5 to about 6.0 comprising a water soluble base in a polymeric solution containing from about 8% to about 20% by Weight of polymeric mixture of water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight together with from about 5% to about 6.7% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular Weight of from about 25,000 to about 150,000.

8. An aqueous composition having a pH of from about 5.5 to about 6.0 comprising a Water soluble base in a polymeric solution containing from about 8% to about 20% by weight of a polymeric mixture of water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight together with from about 5% to about 6.7% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 50,000.

9. An aqueous composition having a pH of from about 4.6 to about 5.0 comprising ammonium hydroxide in a polymeric solution containing from about 8% to about 20% by weight of a polymeric mixture or" a water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight together with from about 5% to about 6.7% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 150,000 and a dichromate light sensitizing agent.

10. An aqueous composition having a pH of from about 4.6 to about 5.0 comprising ammonium hydroxide in a polymeric solution containing from about 8% to about 20% by weight of a polymeric mixture of a water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight together with from about 5% to about 6.7% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 50,000 and a dichromate light sensitizing agent.

11. A photoengraving plate comprising a zinc base with a surface coating of a thin photosensitive layer comprising a light sensitizing agent dispersed in a polymer mixture comprising water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight and from about 5% to about 10% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 150,000.

12. A photoengraving plate comprising a zinc base with a surface coating of a thin photosensitive layer comprising a dichromate light sensitizing agent dispersed in a polymer mixture comprising water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight and from about 5% to about 10% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular Weight of from about 25,000 to about 150,000.

13. A photoengraviug plate comprising a zinc base with a surface coating of a photosensitive layer from about 0.0002 to about 0.0004 inch thick comprising a light sensitizing agent dispersed in a polymer mixture comprising water soluble polyvinyl alcohol having an ash content of less than 0.6% by Weight and from about 5% to about 10% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 150,000.

14. A photoengraving plate comprising a zinc base with a surface coating of a photosensitive layer from about 0.0002 to about 0.0004 inch thick comprising a dichromate light sensitizing agent dispersed in a polymer mixture comprising water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight and from about 5% to about 10% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular Weight of from about 25,000 to about 150,000.

15. A photoengraving plate comprising a magnesium base with a surface coatingof a thin photosensitive layer comprising a light sensitizing agent dispersed in a polymer mixture comprising water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight and from about 5% to about 6.7% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 150,000.

16. A photoengraving plate comprising a magnesium base with a surface coating of a thin photosensitive layer comprising a dichromate light sensitizing agent dispersed in a polymer mixture comprising water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight and from about 5% to about 6.7% by weight based on the total weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 150,000.

17. A photoengraving plate comprising a magnesium base with a surface coating of a photosensitive layer from about 0.0002 to about 0.0004 inch thick comprising a light sensitizing agent dispersed in a polymer mixture comprising water soluble polyvinyl alcohol having an ash content of less than 0.6% by weight and from about 5% to about 6.7% by weight based on the total Weight of polymer of water soluble polyacrylic acid having a molecular weight of from about 25,000 to about 150,000.

18. A photoengraving plate comprising a magnesium base with a surface coating of a photosensitive layer from about 0.0002 to about 0.0004 inch thick comprising a dichromate light sensitizing agent dispersed in a polymer mixture comprising water soluble polyvinyl alcohol having an ash content of less than 0.6% by Weight and from about 5% to about 6.7% by weight based on the total Weight of polymer of water soluble polyacrylic acid having a References Cited UNITED STATES PATENTS Richardson et al. 26029.6 X

NORMAN G. TORCHIN, Primary Examiner.

J. TRAVIS BROWN, Examiner.

molecular weight of from about 25,000 to about 150,000. 15 RONALD H. SMITH, Assistant Examiner. 

1. AN AQUEOUS COMPOSITION COMPRISING A POLYMERIC SOLUTION CONTAINING FROM ABOUT 8% TO ABOUT 20% BY WEIGHT OF A POLYMERIC MIXTURE OF WATER SOLUBLE POLYVINYL ALCOHOL HAVING AN ASH CONTENT OF LESS THAN 0.6% BY WEIGHT TOGETHER WITH FROM 5% TO ABOUT 10% BY WEIGHT BASED ON THE TOTAL WEIGHT OF POLYMER OF WATER SOLUBLE POLYACRYLIC ACID HAVING A MOLECULAR WEIGHT OF FROM ABOUT 25,000 TO ABOUT 150,000.
 3. AN AQUEOUS COMPOSITION COMPRISING A POLYMERIC SOLUTION CONTAINING FROM ABOUT 8% TO ABOUT 20% BY WEIGHT OF A POLYMERIC MIXTURE OF WATER SOLUBLE POLYVINYL ALCOHOL HAVING AN ASH CONTENT OF LESS THAN 0.6% BY WEIGHT TOGETHER WITH FROM ABOUT 5% TO ABOUT 10% BY WEIGHT BASED ON THE TOTAL WEIGHT OF POLYMER OF WATER SOLUBLE POLYACRYLIC ACID HAVING A MOLECULAR WEIGHT OF FROM ABOUT 25,000 TO ABOUT 150,000 AND A LIGHT SENSITIZING AGENT.
 13. A PHOTOENGRAVING PLATE COMPRISING A ZINC BASE WITH A SURFACE COATING OF A PHOTOSENSITIVE LAYER FROM ABOUT 0.0002 TO ABOUT 0.0004 INCH THICK COMPRISING A LIGHT SENSITIZING AGENT DISPERSED IN A POLYMER MIXTURE COMPRISING WATER SOLUBLE POLYVINYL ALCOHOL HAVING AN ASH CONTENT OF LESS THAN 0.6% BY WEIGHT AND FROM ABOUT 5% TO ABOUT 10% BY WEIGHT BASED ON THE TOTAL WEIGHT OF POLYMER OF WATER SOLUBLE POLYACRYLIC ACID HAVING A MOLECULAR WEIGHT OF FROM ABOUT 25,000 TO ABOUT 150,000. 